Most commercial refrigeration systems operate on a closed-loop cycle in which refrigerant circulates through a compression, condenser, flow-control device such as an expansion valve or capillary tube, evaporator and back to the compressor. The flow-control device is often arranged to maintain the output temperature of the evaporator at a constant value. Due to pressure differential requirements of standard thermostatic expansion valves, there must be sufficient differential pressure between the head pressure and the suction pressure to enable the expansion valves to feed refrigerant into the evaporator. Under normal operating conditions, there is enough pressure at the outlet of the condenser to enable the flow-control device to maintain a normal amount of refrigerant flow and evaporator pressure.
However, under certain ambient conditions, e.g., low ambient temperature, the pressure at the outlet of the condenser may drop thereby reducing the pressure differential across the expansion valve, and therefore, the amount of refrigerant that can flow through the expansion valve. This may cause various problems including a reduction in the evaporator pressure which, in turn, may reduce the system efficiency, cause short cycling, etc. Therefore, it has been mandatory to install a fan cycling switch or a head pressure control valve in the system.
Some attempts have been made to substitute electronically controlled expansion valves including balanced port valves, for mechanical valves, but the results devices have proven unworkable due to problems such as high complexity, bulky construction, and unreliable operation.